1. Field
This application relates to guitar amplifiers and a means to attenuate them.
2. Prior Art
Musicians' amplifiers, and more specifically, guitar amplifiers, have been used for over 50 years to amplify the sound of instruments. In the case of guitar amplifiers, the function of the amplifier has gone far beyond its original purpose of increasing the volume of a basically acoustic instrument and become an integral part of the sound of the electric guitar. Specifically, the loud, distorted sound typified by the large Marshall and Fender tube amplifier stacks that when intentionally overdriven well beyond the tubes original purpose has become synonymous with the sound of rock and roll guitar. However, for purposes of duplicating these sounds at low volumes for use in small venues or for practice, these large amplifiers are wholly impractical. Indeed, for most uses, the large tube amplifiers have been replaced by smaller, lighter tube amplifiers but paying a drastic penalty in sound quality for rock and roll.
Large amplifiers typically use a highly efficient class ab output topology with at least two tubes conducting much less than 360 degrees but more than 180 degrees in a push-pull configuration with feedback. A two-tube amplifier will typically give you 50 watts output power. This type of output stage when driven hard will distort in a fashion characterized by square waves and a sound that is unique to this type of output stage. One remedy is to use lower power output tubes connected in the same configuration but these tubes sound different than the higher power variants and although the power is lower it is still way too loud for most applications. Another remedy is to use a single ended grounded cathode output stage biased class A or close to it. This configuration has a much lower efficiency therefore less power but the sound is nothing like the musical speaker thrashing square waves of the push pull configuration due to its high output impedance and rounded and flabby distortion characteristics.
Another remedy is to use an attenuator between the output stage and the speaker load of a tube output stage. Sholz U.S. Pat. Nos. 4,363,934 and 4,143,245. Due to the limitation of having to maintain a constant impedance across the load or else imminent tube destruction will occur it necessitates using series and shunting voltage dividers. The end result is the speaker being robbed of tone and the musician complaining that the feel of the overdriven output stage is gone. This flattening out of the tone occurs because the current is being shunted from the speaker and the vital output-stage speaker interaction is gone. This interaction can be felt as a fatness or a resistance while playing that is unmistakable to an experienced musician, and it is one of the most important considerations to the inspiration of the musician and the tone that is achieved.
Yet still another option is to introduce the distortion in the stages prior to the output stage using either vacuum tubes or solid state devices. This leaves much to be desired.
None of these techniques, however, address the critical parameters for emulating loud, distorted classic sound in a small tube guitar amplifier with or without an attenuator therefore, it is the object of the presently preferred embodiment to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the presently preferred embodiment to provide a new and improved output stage for a low powered guitar amplifier that remedies the inherent problems of the single ended grounded cathode amplifier while maintaining the attributes of the high powered push-pull amplifiers.
Another objective is to provide a new and improved attenuator circuit for a musical instrument amplifier that when used in conjunction with a cathode follower final output power stage remedies all of the deficiencies of the prior art regardless of configuration or topology.